Locking mast assemblies are provided with a plurality of telescoping tubes beginning with a first, outer tube that receives pressurized air through the base and which is open at the top where an inner, concentric tube is located. This inner tube allows for the passage of air through its base and into additional tubes that are provided in order to obtain a desired height when the mast is fully extended. The last, innermost tube is sealed at the bottom to contain air which allows the mast to be extended as air is added.
Each tube is provided with a locking collar that maintains an adjacent inner tube in the extended position so that the air pressure supply can be removed. Such locking assemblies are known and commonly introduce one or more locking pins circumferentially around the base area of an inner extended tube to engage recesses, grooves, keys and the like.
In order to maintain the portability of the telescoping mast, particularly mobility from location to location and ease of raising and lowering, it is desirably constructed of light weight metal such as aircraft aluminum. Although the mast is not exceedingly heavy, it is necessary to employ a locking pin of greater strength such as steel and preferably stainless steel so that the pin is not sheared or prematurely weared. However, supporting the weight of distal segments by pins alone which are, in turn, in mating engagement with aluminum will cause deformation of the aluminum via bending, peening, even tearing, such that the mast may not fully extend, it may be subject to undue axial rotation and in either event extension and lowering can become impaired eventually leading to failure of the mast.
Possible solutions to this problem have involved reinforcement of the pin engaging area by compatible metal such as steel. Nevertheless, this gives rise to a new set of problems because of the difficulty of affixing the reinforcements to the aluminum tubes. Factors to be considered are that the telescoping engagement must not be impaired, i.e., the reinforcement must fit within a concentric lower tube of greater diameter and, the pneumatic integrity must not be breached. Welding the reinforcement to the tube affixes the two together well but weakens the tube which can lead to failure of the assembly. Tack welds, coupled with the use of adhesives, avoids weakening the tube but these can fail.
One early patent of which we are aware, U.S. Pat. No. 2,708,493, is directed toward a portable antenna mast that can be raised hydraulically and which carries a locking assembly. The latter employs a stop collar affixed to the outboard end of a telescoping inner tube section and spring-biased plungers that are positioned as soon as the stop collar passes thereover.
Thus, despite the fact that a variety of locking pin arrangements have been developed, the problems discussed hereinabove as well as others, have not been satisfactorily eliminated until now.